Direct CP Violation in K→μ+μ-.

A rare decay KL→µ+µ- has been measured precisely, while a rare decay KS→µ+µ- will be observed by an upgrade of the LHCb experiment. Although both processes are almost CP-conserving decays, we point out that an interference contribution between KL and KS in the kaon beam emerges from a genuine direct CP violation. It is found that the interference contribution can change KS→µ+µ- standard-model predictions at O(60%). We also stress that an unknown sign of A(KL→γγ) can be determined by a measurement of the interference, which can much reduce a theoretical uncertainty of B(KL→µ+µ-). We also investigate the interference in a new physics model, where the ε'K/εK tension is explained by an additional Z-penguin contribution. [ABSTRACT FROM AUTHOR]